(1) Field of the Invention
The present invention relates to a coating composition capable of forming a titanium oxide film showing good properties in photoactivity, antibacterial properties, hydrophilic properties, pollution resistance, hazeproofness, gas decomposition properties, deodorizing properties, water treatment properties, energy transformation properties, decoloring properties, etc., a method of preparing the coating composition, and a method of forming the inorganic film.
(2) Description of Background Art
The titanium oxide film-forming methods known in the art may include, for example, (1) a method of coating a titanium oxide sol onto a substrate, followed by sintering, (2) a method of coating an aqueous solution of titanium chloride or titanium sulfate onto a substrate, followed by heat treating, (3) a plasma spray coating method which comprises fusing titanium oxide solid particles in the presence of plasma generated in atmosphere, followed by spraying onto a substrate, (4) a spatter spraying method of spattering the oxide as a target under vacuum, followed by spraying onto a substrate, (5) a chemical vapor deposition method of volatilizing an organic metal compound or the like, followed by decomposing in an electric furnace, and forming a film onto a substrate, (6) a sol-gel method of coating a sol obtained by hydrolysis of a metal alkoxide onto a substrate, followed by sintering, and the like.
However, the above methods (1) to (6) have such drawbacks that the method (1) is such that a film thickness of 0.1 xcexcm or more may develop cracks and peelings, resulting in showing poor film-forming properties, and sintering at a temperature of hundreds is required, resulting in requiring much time and labor, the method (2) is such that adverse effects of thermal decomposition products on the substrate and necessity of sintering at a temperature of hundreds require much time and labor, the method (3) is unable to form a fine film and results poor adhesion properties, the methods (4) and (5) are such that a satisfactory film may be obtained under vacuum, so that a vacuumizing reactor may be required, and that a film-forming speed may be so low that heating the substrate at a temperature of hundreds to obtain a fine film, and the method (6) is such that the acid, alkali and organic compounds contained in the sol of the method (6) may produce problems of corrosion of a substrate, and that removal of the organic compounds requires heating at 400xc2x0 C. or higher.
In addition to the above methods, the methods free of the step of sintering at high temperatures may include (7) a method of forming a titanium oxide film from an aqueous fluoride solution of titanium oxide and boric acid, (8) a method which comprises precipitating a titanium hydroxide gel from an aqueous solution of titanium chloride or titanium sulfate and an alkali solution such as ammonia, caustic soda and the like, followed by separating the titanium hydroxide gel by decantation, thoroughly washing with water, and adding hydrogen peroxide water (see Japanese Patent Application Laid-Open No. 71418/97), and the like.
However, the method (7) has such drawbacks that formation of the film requires much time, and handling of the fluoride is troublesome, and the method (8) has such drawbacks that the method (8) comprises complicated steps, removal of the alkali salt is troublesome, inclusion of other metals may make it difficult to obtain a high purity film, difficulty in control of pH results films different from each other depending on variation of pH, and so forth.
It is an object of the present invention to provide a coating composition suitable for use in forming an inorganic film, and free of byproducts such as alkali salt, fluoride and the like as in the prior art so as to show good properties in storage stability, film performances, etc.
It is another object of the present invention to provide a coating composition for use in forming an inorganic film, particularly a titanium oxide film, a preparation method of the coating composition and a method of forming an inorganic film, particularly a titanium oxide film, so that the titanium oxide film may be formed at such a low temperature that the resulting titanium oxide film may show characteristic film performances such as photoactivity, antibacterial properties, hydrophilic properties, pollution resistance, hazeproofness, gas decomposition properties, deodorizing properties, water treatment, energy transformation, decoloring properties and the like.
That is, the present invention provides an inorganic film-forming coating composition prepared by reacting a titanium monomer and/or a low condensation product thereof containing a group hydrolized to form hydroxyl group with a hydrogen peroxide water, a heat modified, inorganic film-forming coating composition prepared by heat treating at 80xc2x0 C. to 100xc2x0 C. or autoclaving at 80xc2x0 C. to 200xc2x0 C. the inorganic film-forming coating composition to form titanium oxide fine particles, a method of preparing the inorganic film-forming coating composition, which method comprises adding the titanium monomer and/or the low condensation product thereof containing the group hydrolized to form hydroxyl group into the hydrogen peroxide water for reacting preferably at a temperature in the range of 10 to 70xc2x0 C., a method of forming an inorganic film, which method comprises coating the inorganic film-forming coating composition or the heat-modified inorganic film-forming coating composition onto the substrate, or impregnating the substrate with the coating composition, followed by drying or heat treating to form an inorganic film preferably showing good properties in photoactivity, antibacterial properties, hydrophilic properties, pollution resistance, hazeproofness, gas decomposition properties, deodorizing properties, water treatment properties, energy transformation properties and decoloring properties.
The inorganic film-forming coating composition in the present invention is obtained by reacting a titanium monomer and/or a low condensation product thereof containing a group hydrolized to form hydroxyl group with a hydrogen peroxide water.
The titanium monomer containing the group hydrolized to form hydroxyl group may preferably include tetraalkoxytitanium represented by the general formula: Ti(OR)4, wherein R is same or different, and represents alkyl group having 1 to 5 carbon atoms. Examples of the alkyl group may include methyl group, ethyl group, n-propyl group, iso-propyl group, n-butyl group, iso-butyl group, sec-butyl group, tert-butyl group, and the like.
The low condensation product of the titanium monomer containing the group hydrolized to form hydroxyl group may preferably include compounds obtained by subjecting the titanium monomer represented by the above general formula: Ti(OP)4 to condensation reaction and having a degree of condensation of 2 to 30, particularly 2 to 10.
A mixing ratio of the titanium monomer and/or the low condensation product thereof (hereinafter may be referred to as a hydrolizable titanium compound) containing the group hydrolized to form hydroxyl group to the hydrogen peroxide water is in the range of 0.1 to 100 parts by weight, particularly 1 to 20 parts by weight as hydrogen peroxide per 10 parts by weight of the hydrolizable titanium compound. An amount less than 0.1 part by weight of the hydrogen peroxide results incomplete formation of a chelate compound, resulting in developing hazes and precipitates. On the other hand, when more than 100 parts by weight, unreacted hydrogen peroxide may remain so that a dangerous active oxygen may be discharged during storage.
A hydrogen peroxide content in the hydrogen peroxide water may not particularly be limited, but preferably be in the range of 3 to 30% by weight from the standpoints of easiness of handling and a solid content of the resulting solution regarding to coating workability.
The inorganic film-forming coating composition in the present invention may be prepared by reacting the hydrolizable titanium compound with the hydrogen peroxide water at a reaction temperature in the range of 10 to 70xc2x0 C. for 10 minutes to 20 hours.
The inorganic film-forming coating composition in the present invention may be obtained in such a manner that the reaction of the hydrolizable titanium compound with the hydrogen peroxide water results hydrolysis of the hydrolizable titanium compound to form a hydroxyl group-containing titanium compound, followed by coordinating the hydroxyl group-containing titanium compound with hydrogen peroxide, wherein the hydrolysis reaction and the coordination may be almost simultaneously take place, to form a chelate solution showing high stability at room temperature and a long term storage stability. On the other hand, a titanium hydroxide gel used in the preparation of the titanium oxide film in the art is partly three dimensionalized through a Ti-O-Ti linkage, and is essentially different from the above reaction product of the hydroxyl group-containing titanium compound with the hydrogen peroxide water in composition and stability.
The inorganic film-forming coating composition in the present invention may be heat treated at 80xc2x0 C. to 100xc2x0 C., or autoclaved at 80xc2x0 C. to 200xc2x0 C. to obtain a titanium oxide dispersion containing ultrafine particles of crystallized titanium oxide. When lower than 80xc2x0 C., crystallization of titanium oxide may not satisfactorily proceed. The above titanium oxide dispersion may also be used as a titanium oxide film-forming solution.
The inorganic film-forming coating composition in the present invention may be coated onto a substrate, followed by drying or heat treating at a low temperature to form a fine titanium oxide film showing good adhesion properties. A titanium oxide film may be formed at a low temperature, but a heat treating temperature of 200xc2x0 C. to 500xc2x0 C., preferably 200xc2x0 C. to 350xc2x0 C. is preferable for the purpose of improving adhesion properties. One coating makes it possible to form a titanium oxide film having a thickness of 1 xcexcm or more, and showing good adhesion properties without being separated, but the film thickness is usually in the range of 0.001 to 10 xcexcm, particularly 0.1 to 1 xcexcm.
The inorganic film-forming coating composition in the present invention is such that a heat treatment at a temperature lower than 200xc2x0 C. forms a non-crystalline titanium oxide film containing some hydroxyl group, whereas when 200xc2x0 C. or higher, a fine crystalline titanium oxide film is formed.
Coating only of the titanium oxide dispersion obtained by the heat treatment at 80xc2x0 C. to 100xc2x0 C. makes it possible to form a crystalline titanium oxide film so as to be useful as a coating composition for use in a substrate incapable of heat treating, resulting in being applicable to various uses as above described and in making it possible to form a film having a relatively high density and showing good adhesion properties at low temperatures.
The above film prepared by drying only may show water resistance and impregnating properties so as to be impregnated with a solution of other compound, resulting in that the following heat treatment makes it possible to form such a composite material that the other compound may be deposited or dispersed into the titanium oxide film. The other compound may include, for example, other metal compounds.
Other pigments and sols may be added and dispersed into the inorganic film-forming coating composition. Other pigments and sols to be added may include, for example, known titanium oxide sols, titanium oxide powder, etc.
The substrate used in the present invention may include any materials resistant to heat treatment depending on uses, for example, metal, pottery, plastics, fiber, glass, concrete and the like, and may also include the interior surface of a porous material, the surface of a powder to be treated.
Many titanium oxide films obtained in the present invention may show activity on photo-irradiation. For example, photooxidation of nitrogen oxide, etc. may result nitric acid.
The titanium oxide film obtained in the present invention shows, in addition to photoactivity, good properties in heat resistance, corrosion resistance, and may be used as a heat resistant and corrosion resistant film.
Taking advantage of photo-activity of the titanium oxide film, the film may be applicable to various fields such as decomposition and removal of environmental pollutants in air and water, hydrophilizing treatment, antibacterial treatment, deodorizing treatment, hazeproof treatment, water treatment, energy transformation and the like.
The inorganic film-forming coating composition of the present invention is a high purity chelate solution free of byproducts such as an alkali salt, fluoride and the like as in the prior art, and shows good storage stability and is capable of forming a titanium oxide film showing good film performances.
The present invention can provide an improved preparation method of the above coating composition without including complicated steps such as a step of treating by products and the like as in the prior art.
The titanium oxide film formed from the above chelate solution may easily be formed at low temperatures and may show characteristic properties of the film itself, resulting in being applicable to the fields such as corrosion resistant fields, fields taking advantage of photocatalitic activity, and the like.